Next Js Cannot fill database with data using an internal POST API

<div className={styles.Panier}>
     <p onClick={()=>Quantity>0?NewItem(data,Quantity):console.log("Quantity must be more 
     than 0")}>Add To Cart</p>
</div>

When Clicking on the that div the following NewItem function gets executed:

    let NewItem = async (data,Qte) => {
        const response = await fetch('/api/UpdateCart',{
            method: "POST",
            body: JSON.stringify(
                {
                    Qte,
                    data,
                    cartId: Cookies.get('cart')
                }
            ),
            headers:{
                'Content-Type': 'application/json; charset=utf8'
            },
        });
        console.log(response);
    };

This normally should go to the api in '/api/UpdateCart' (I'm using prisma btw):

import { PrismaClient } from "@prisma/client";


export default async function handler(req, res)
{
    let prisma = new PrismaClient();
    let Id = req.body.Id;
    let Qte = req.body.Qte;
    let cartId = req.body.cartId;
    //Creates the new item and adds it to the cart
    let newItem = await prisma.item.create({
        data: {
            ProductId: Id,
            Quantity: Qte,
            CartId: cartId
        }
    });
    return res.status(200).json(newItem);
}

Now the problem is that when i try the endpoint in postman it works like a charm, but when i use access it from the NewItem function that i mentionned before, I get a 500 server error in the following way:

POST http://localhost:3000/api/UpdateCart 500 (Internal Server Error)

Response {type: 'basic', url: 'http://localhost:3000/api/UpdateCart', redirected: false, status: 500, ok: false, …}
body
: 
(...)
bodyUsed
: 
false
headers
: 
Headers {}
ok
: 
false
redirected
: 
false
status
: 
500
statusText
: 
"Internal Server Error"
type
: 
"basic"
url
: 
"http://localhost:3000/api/UpdateCart"
[[Prototype]]
: 
Response

I hope that u can help me and thank you for your time

After more than a day of searching i found out that the problem was actually in API code since i was having one-to-many relationships I was supposed to 'connect' the full 'Cart' and 'Product' records into my item record.

1. This is the API code before the solution:

import { PrismaClient } from "@prisma/client";


export default async function handler(req, res)
{
   let prisma = new PrismaClient();
   let Id = req.body.Id;
   let Qte = req.body.Qte;
   let cartId = req.body.cartId;
   //Creates the new item and adds it to the cart
   let newItem = await prisma.item.create({
       data: {
           ProductId: Id,
           Quantity: Qte,
           CartId: cartId
       }
   });
   return res.status(200).json(newItem);
}

2. This is the Correct API code:

import { PrismaClient } from "@prisma/client";
const prisma = new PrismaClient();

export default async (req, res) => {
  
  let PrId = req.body.prodId;
  let Qte = req.body.Qte;
  let newItem = await prisma.item.create({
    data:{
      Quantity: Qte,
      Product:{
        connect:{
          Id: PrId,
        },
      }, 
      Cart:{
        connect:{
          Id: req.body.cartId
        }
      }
    },
  });

  return res.status(200).json(newItem);
};